MEMS sensor

ABSTRACT

The MEMS sensor according to the present invention includes a diaphragm. In the diaphragm, an angle formed by two straight lines connecting supporting portions and the center of a main portion with one another respectively is set to satisfy the relation of the following formula (1): (A2/A1)/(B2/B1)≧1  (1) A2: maximum vibrational amplitude of the diaphragm in a case of working a physical quantity of a prescribed value on the diaphragm A1: maximum vibrational amplitude of the diaphragm in a case of working the physical quantity on the diaphragm in an omitting structure obtained by omitting one of the supporting portions from the diaphragm B2: maximum stress caused in the diaphragm in the case of working the physical quantity on the diaphragm B1: maximum stress caused in the diaphragm in the case of working the physical quantity on the diaphragm in the omitting structure

This is a Continuation of U.S. application Ser. No. 12/485,696, filedJun. 16, 2009, now the U.S. Pat. No. 8,039,911, the subject matter ofwhich is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to various sensors (MEMS sensors)manufactured by an MEMS (Micro Electro Mechanical Systems) technique.

2. Description of Related Art

An MEMS sensor, having been recently loaded on a portable telephone orthe like, is watched with interest. For example, a silicon microphone(an Si microphone) is a typical MEMS sensor.

The silicon microphone includes a silicon substrate. A through-holepassing through the silicon substrate in the thickness direction isformed in the central portion of the silicon substrate. A diaphragm madeof polysilicon is provided on the silicon substrate to be opposed to thethrough-hole. The diaphragm integrally has a main portion and a singlesupporting portion supporting the main portion. The main portion isgenerally circular in plan view. The supporting portion is in the formof an arm extending sideward from the peripheral edge of the mainportion, and the forward end portion thereof is fixed to the siliconsubstrate. Thus, the diaphragm is vibratile in a direction opposed tothe surface of the silicon substrate in such a one-point support statethat the main portion is supported by the single supporting portion. Aback plate is arranged on a side of the diaphragm opposite to thesubstrate at a small interval from the diaphragm. The diaphragm and theback plate form a capacitor having the diaphragm and the back plate ascounter electrodes.

When the diaphragm vibrates due to a sound pressure (sound wave) while aprescribed voltage is applied to the capacitor (between the diaphragmand the back plate), the capacitance of the capacitor changes, andvoltage fluctuation between the diaphragm and the back plate resultingfrom the change of the capacitance is output as a sound signal.

The sensitivity of the silicon microphone is improved as the vibrationalamplitude (displacement in a direction orthogonal to a sound pressureinput surface) of the diaphragm with respect to the same sound pressureis increased. If the diaphragm remarkably vibrates and excessive stressis caused in the diaphragm, however, the diaphragm is broken due to thestress.

The vibrational amplitude (vibratility) of the diaphragm with respect tothe same sound pressure can be reduced by increasing the number ofsupporting portions, for example. Depending on the arrangement of aplurality of supporting portions, however, the vibrational amplitude ofthe diaphragm with respect to the same sound pressure is extremelyreduced to remarkably lower the sensitivity of the silicon microphone.When the diaphragm is supported by a plurality of supporting portions,therefore, the supporting portions must be arranged with dueconsideration.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide an MEMSsensor having a plurality of supporting portions arranged on properpositions.

An MEMS sensor according to an aspect of the present invention includesa substrate and a diaphragm provided on the substrate for vibrating dueto the action of a physical quantity. The diaphragm integrally has amain portion arranged on the substrate in a floating state andsupporting portions extending in a direction along the surface of thesubstrate from two positions in the peripheral edge of the main portion.An angle formed by two straight lines connecting the supporting portionsand the center of the main portion with one another respectively is setto satisfy the relation of the following formula (1):(A2/A1)/(B2/B1)≧1  (1)

A2: maximum vibrational amplitude of the diaphragm in a case of workinga physical quantity of a prescribed value on the diaphragm

A1: maximum vibrational amplitude of the diaphragm in a case of workingthe physical quantity of the prescribed value on the diaphragm in astructure obtained by omitting one of the supporting portions from thediaphragm

B2: maximum stress caused in the diaphragm in the case of working thephysical quantity of the prescribed value on the diaphragm

B1: maximum stress caused in the diaphragm in the case of working thephysical quantity of the prescribed value on the diaphragm in thestructure obtained by omitting one of the supporting portions from thediaphragm

Acceleration or a sound pressure can be illustrated as the physicalquantity.

According to the structure, the diaphragm vibrating due to the action ofthe physical quantity is provided on the substrate. The diaphragmintegrally has the main portion and the two supporting portions. Themain portion is supported by the two supporting portions, and arrangedon the substrate in the floating state. The two supporting portionsextend in the direction along the surface of the substrate fromdifferent portions of the peripheral edge of the main portionrespectively.

When the main portion is supported by the two supporting portions, themaximum vibrational amplitude A2 of the diaphragm in the case where thephysical quantity of the prescribed value acts on the diaphragm and themaximum stress B2 caused in the diaphragm at this time depend on thearrangement of the two supporting portions. The maximum vibrationalamplitude A2 is smaller than the maximum vibrational amplitude A1 of thediaphragm in the case where the physical quantity of the prescribedvalue acts on the diaphragm having the structure supporting the mainportion with one supporting portion. Further, the maximum stress B2 issmaller than the maximum stress B1 caused in the diaphragm in the casewhere the physical quantity of the prescribed value acts on thediaphragm having the structure supporting the main portion with onesupporting portion. The diaphragm is hardly broken by stress as themaximum stress B2 is reduced.

If the two supporting portions are so arranged that the reduction ratioof the maximum vibrational amplitude A2 with respect to the maximumvibrational amplitude A1 exceeds the reduction ratio of the maximumstress B2 with respect to the maximum stress B1, however, thesensitivity of the MEMS sensor (vibratility of the diaphragm) isdisadvantageously reduced despite the advantage of preventing breakageof the diaphragm attained by stress reduction.

Therefore, the angle formed by the two straight lines connecting thesupporting portions and the center of the main portion with one anotherrespectively is set to satisfy the relation (A2/A1)/(B2/B1)≧1. Thus, thetwo supporting portions are arranged on proper positions, and thereduction ratio of the maximum vibrational amplitude A2 with respect tothe maximum vibrational amplitude A1 does not exceed the reduction ratioof the maximum stress B2 with respect to the maximum stress B1.Consequently, remarkable reduction of the sensitivity of the MEMS sensorcan be prevented while breakage of the diaphragm resulting from stresscan be suppressed.

An MEMS sensor according to another aspect of the present inventionincludes a substrate and a diaphragm provided on the substrate forvibrating due to the action of a physical quantity. The diaphragmintegrally has a main portion arranged on the substrate in a floatingstate and supporting portions extending in a direction along the surfaceof the substrate from not less than three positions in the peripheraledge of the main portion. Not less than three supporting portions arearranged to satisfy the relation of the following formula (2):(Ap/A1)/(Bp/B1)≧1  (2)

Ap: maximum vibrational amplitude of the diaphragm in a case of workinga physical quantity of a prescribed value on the diaphragm

A1: maximum vibrational amplitude of the diaphragm in a case of workingthe physical quantity of the prescribed value on the diaphragm in astructure obtained by omitting the remaining supporting portions otherthan one of the supporting portions from the diaphragm

Bp: maximum stress caused in the diaphragm in the case of working thephysical quantity of the prescribed value on the diaphragm

B1: maximum stress caused in the diaphragm in the case of working thephysical quantity of the prescribed value on the diaphragm in thestructure obtained by omitting the remaining supporting portions otherthan one of the supporting portions from the diaphragm

According to the structure, the diaphragm vibrating due to the action ofthe physical quantity is provided on the substrate. The diaphragmintegrally has the main portion and not less than three supportingportions. The main portion is supported by not less than threesupporting portions, and arranged on the substrate in the floatingstate. Not less than three supporting portions extend in the directionalong the surface of the substrate from different portions of theperipheral edge of the main portion respectively. Not less than threesupporting portions are arranged to satisfy the relation(Ap/A1)/(Bp/B1)≧1. Thus, not less than three supporting portions arearranged on proper positions, and the reduction ratio of the maximumvibrational amplitude Ap with respect to the maximum vibrationalamplitude A1 does not exceed the reduction ratio of the maximum stressBp with respect to the maximum stress B1. Consequently, remarkablereduction of the sensitivity of the MEMS sensor can be prevented whilebreakage of the diaphragm resulting from stress can be suppressed.

The foregoing and other objects, features and effects of the presentinvention will become more apparent from the following detaileddescription of the embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a silicon microphone accordingto an embodiment of the present invention.

FIG. 2 is a schematic plan view of a diaphragm shown in FIG. 1.

FIG. 3 is a schematic plan view of a silicon microphone according toanother embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a schematic sectional view of a silicon microphone accordingto an embodiment of the present invention.

A silicon microphone 1 is a sensor (an MEMS sensor) manufactured by theMEMS technique. The silicon microphone 1 includes a substrate 2 made ofsilicon. A through-hole 3 having a trapezoidal sectional shape narrowedtoward the surface (widened toward the rear surface) is formed in thecentral portion of the substrate 2.

A first insulating film 4 is stacked on the substrate 2. The firstinsulating film 4 is made of silicon oxide, for example.

A second insulating film 5 is stacked on the first insulating film 4.The second insulating film 5 is made of PSG (Phospho Silicate Glass),for example.

Portions of the first insulating film 4 and the second insulating film 5located on the through-hole 3 and a portion (hereinafter referred to asa “through-hole peripheral portion”) of the surface (the upper surface)of the substrate 2 around the through-hole 3. Thus, the through-holeperipheral portion is exposed from the first insulating film 4 and thesecond insulating film 5.

A diaphragm 6 is provided above the substrate 2. The diaphragm 6 is madeof polysilicon doped with an impurity to be provided with conductivity,for example. The diaphragm 6 integrally has a main portion 7 and twosupporting portions 8.

The main portion 7 is circular in plan view, opposed to the through-hole3 and the through-hole peripheral portion, and arranged in a statefloating above the through-hole peripheral portion. A plurality ofprotruding lower stoppers 9 for preventing the main portion 7 and thethrough-hole peripheral portion from adhesion are formed on the lowersurface (the surface opposed to the through-hole peripheral portion) ofthe main portion 7.

The two supporting portions 8 extend in a direction (a sidewarddirection) along the surface of the substrate 2 from two positions onthe peripheral edge of the main portion 7 respectively. The forward endportion of each supporting portion 8 enters the space between the firstinsulating film 4 and the second insulating film 5, so that thesupporting portion 8 is cantilever-supported by the first insulatingfilm 4 and the second insulating film 5. The two supporting portions 8support the main portion 7, whereby the diaphragm 6 is enabled tovibrate in a direction A1 opposed to the surface of the substrate 2 in atwo-point support state.

A back plate 10 is provided above the diaphragm 6. The back plate 10 hasa circular outer shape smaller in diameter than the main portion 7 ofthe diaphragm 6 in plan view, and is opposed to the main portion 7through a gap. The back plate 10 is made of polysilicon doped with animpurity to be provided with conductivity, for example.

The outermost surface of the silicon microphone 1 is covered with athird insulating film 11. The third insulating film 11 is formed tocover the upper surfaces of the first insulating film 4 and the backplate 10 and to surround the side portion of the diaphragm 6 at aninterval from the peripheral edge of the diaphragm 6. Thus, a space 12partitioned by the third insulating film 11 is formed on the substrate2, and the main portion 7 of the diaphragm 6 is arranged in the space 12in a state not in contact with the substrate 2 and the third insulatingfilm 11.

A large number of small holes 13 are formed in the back plate 10 and thethird insulating film 11 to continuously pass through the same. Thethird insulating film 11 enters partial holes 13, and protruding upperstoppers 14 protruding downward beyond the lower surface (the surfaceopposed to the diaphragm 6) of the back plate 10 are formed on theportions of the third insulating film 11 entering the holes 13. Theupper stoppers 14 are so formed that the diaphragm 6 is prevented fromcoming into contact with the back plate 10 upon vibration of thediaphragm 6.

A plurality of communication holes 15 are formed in the third insulatingfilm 11 around the back plate 10 in a circularly aligned manner.

The diaphragm 6 and the back plate 10 form a capacitor having thediaphragm 6 and the back plate 10 as counter electrodes. A prescribedvoltage is applied to the capacitor (between the diaphragm 6 and theback plate 10). When the diaphragm 6 vibrates due to a sound pressure(sound wave) in this state, the capacitance of the capacitor changes,and voltage fluctuation between the diaphragm 6 and the back plate 10resulting from the change of the capacitance is extracted (output) as asound signal.

FIG. 2 is a schematic plan view of the diaphragm 6.

The two supporting portions 8 of the diaphragm 6 are arranged on twopositions separated by an angle α from each other around the center ofthe main portion 7 respectively. In other words, the two supportingportions 8 are so arranged that a straight line L1 connecting one of thesupporting portions 8 and the center of the main portion 7 with eachother and a straight line L2 connecting the other supporting portion 8and the center of the main portion 7 with each other form the angle α.

The angle α is set to satisfy the following formula (1):(A2/A1)/(B2/B1)≧1  (1)

A2: maximum vibrational amplitude of the diaphragm 6 in a case ofinputting a prescribed sound pressure in the diaphragm 6

A1: maximum vibrational amplitude of a diaphragm having a main portion 7supported by one supporting portion 8 in a case of inputting theprescribed sound pressure in the diaphragm

B2: maximum stress caused in the diaphragm 6 in the case of inputtingthe prescribed sound pressure in the diaphragm 6

B1: maximum stress caused in the diaphragm having the main portion 7supported by one supporting portion 8 in the case of inputting theprescribed sound pressure in the diaphragm

<Simulation>

The inventor of the present invention has conducted a simulation forexamining the relation between the angle α, the maximum vibrationalamplitude of the diaphragm 6 and the maximum stress caused in thediaphragm 6.

In the simulation, “IntelliSuite (registered trademark)” by IntelliSenseCorporation was employed as a simulator. A diaphragm (i.e., thediaphragm 6) having such a structure (a two-point support structure)that a main portion 7 is supported by two supporting portions 8 and adiaphragm having such a structure (a one-point support structure) that amain portion 7 is supported by a supporting portion 8 were assumed. Thematerial for each diaphragm is polysilicon deposited by LPCVD (LowPressure Chemical Vapor Deposition). Each main portion 7 is 600 μm indiameter by 1 μm in thickness. Each supporting portion 8 is 30 μm by 100μm by 1 μm in size. The following values were input in the simulator asthe physical properties of each diaphragm:

-   -   Density: 2.3 [g/cm³]    -   Thermal expansion coefficient: 20 [10E-7/° C.]    -   Resistivity: 1 [Ω·cm]    -   Thermal conductivity: 1.5 [W/cm/° C.]    -   Specific heat: 0.71 [J/g/° C.]    -   Young's modulus: 160 [GPa]    -   Poisson's ratio: 0.226    -   Dielectric constant: 1

1 Pa was input in the main portion 7 of the diaphragm having theone-point support structure, to check the maximum vibrational amplitudeA1 of the diaphragm and the maximum stress B1 caused in the diaphragm.In the diaphragm 6 having the two-point support structure, the angle αshown in FIG. 2 was set to 7.5°, 10°, 12.5°, 15°, 20°, 30°, 40°, 45°,50°, 60°, 75°, 90°, 105°, 120°, 135°, 150°, 165° and 180° and 1 Pa wasinput in the main portion 7 every angle, to check the maximumvibrational amplitude A2 of the diaphragm 6 and the maximum stress B2caused in the diaphragm 6. Further, the ratio (A2/A1) of the maximumvibrational amplitude A2 to the maximum vibrational amplitude A1 and theratio (B2/B1) of the maximum stress B2 to the maximum stress B1 wereobtained, to obtain the value of the left side (A2/A1)/(B2/B1) of theformula (1).

Table 1 shows the results of the simulation.

TABLE 1 Maximum Vibrational Amplitude Maximum Stress Percent PercentRatio to Ratio to One-Point One-Point (A2/A1)/ Angle α A2 [um] SupportB2 [Mpa] Support (B2/B1) 7.5 10.63 54.5 12.97 49.8 1.09 10 10.45 53.612.92 49.6 1.08 12.5 10.3 52.8 12.85 49.3 1.07 15 10.17 52.1 12.76 49.01.06 20 9.92 50.8 12.69 48.7 1.04 30 9.35 47.9 13.14 50.4 0.95 40 8.6944.5 13.27 50.9 0.87 45 8.33 42.7 13.23 50.8 0.84 50 7.96 40.8 13.1150.3 0.81 60 7.18 36.8 12.72 48.8 0.75 75 5.98 30.7 11.77 45.2 0.68 904.83 24.8 10.54 40.5 0.61 105 3.77 19.3 9.15 35.1 0.55 120 2.82 14.5 7.729.6 0.49 135 2.01 10.3 6.27 24.1 0.43 150 1.32 6.8 4.89 18.8 0.36 1650.76 3.9 3.57 13.7 0.28 180 0.31 1.6 2.62 10.1 0.16 One-Point A1 = 19.51— B1 = 26.05 — — Support

As understood from the results of the simulation, the maximumvibrational amplitude A2 and the maximum stress B2 depend on thearrangement of the two supporting portions 8 in the diaphragm 6. Themaximum vibrational amplitude A2 is smaller than the maximum vibrationalamplitude A1. Further, the maximum stress B2 is smaller than the maximumstress B1. The diaphragm 6 is hardly broken by stress as the maximumstress B2 is reduced. If the two supporting portions 8 are so arrangedthat the reduction ratio of the maximum vibrational amplitude A2 withrespect to the maximum vibrational amplitude A1 exceeds the reductionratio of the maximum stress B2 with respect to the maximum stress B1,however, the sensitivity of the silicon microphone 1 (vibratility of thediaphragm 6) is disadvantageously reduced despite the advantage ofpreventing breakage of the diaphragm 6 attained by stress reduction.

Therefore, the angle α formed by the two straight lines L1 and L2 shownin FIG. 2 is set to satisfy the relation (A2/A1)/(B2/B1)≧1. In a modelset in the simulation, for example, the angle α is set in the range ofabout 7.5° to 20° from the results of the simulation. Thus, the twosupporting portions 8 are arranged on proper positions, and thereduction ratio of the maximum vibrational amplitude A2 with respect tothe maximum vibrational amplitude A1 does not exceed the reduction ratioof the maximum stress B2 with respect to the maximum stress B1.Consequently, remarkable reduction of the sensitivity of the siliconmicrophone 1 can be prevented while breakage of the diaphragm 6resulting from stress can be suppressed.

While the main portion 7 is supported by the two supporting portions 8in the aforementioned structure, the main portion 7 may alternatively besupported by not less than three supporting portions 8, as shown in FIG.3. In this case, not less than three supporting portions 8 are arrangedto satisfy the relation of the following formula (2):(Ap/A1)/(Bp/B1)≧1  (2)

Ap: maximum vibrational amplitude of a diaphragm having not less thanthree supporting portions 8 in a case of inputting a prescribed soundpressure in the diaphragm

A1: maximum vibrational amplitude of the diaphragm having the mainportion 7 supported by one supporting portion 8 in a case of inputtingthe prescribed sound pressure in the diaphragm

Bp: maximum stress caused in the diaphragm having not less than threesupporting portions 8 in the case of inputting the prescribed soundpressure in the diaphragm

B1: maximum stress caused in the diaphragm having the main portion 7supported by one supporting portion 8 in the case of inputting theprescribed sound pressure in the diaphragm.

Thus, not less than three supporting portions 8 are arranged on properpositions and the reduction ratio of the maximum vibrational amplitudeAp with respect to the maximum vibrational amplitude A1 does not exceedthe reduction ratio of the maximum stress Bp with respect to the maximumstress B1, similarly to the structure (the structure according to theaforementioned embodiment) having the main portion 7 supported by thetwo supporting portions 8. Consequently, remarkable reduction of thesensitivity of the silicon microphone 1 can be prevented while breakageof the diaphragm 6 resulting from stress can be suppressed.

While the silicon microphone has been employed as an example of the MEMSsensor, the present invention is not restricted to the siliconmicrophone, but applicable to an acceleration sensor for detectingacceleration of a substance or a gyro sensor for detecting angularvelocity of a substance.

While the present invention has been described in detail by way of theembodiments thereof, it should be understood that these embodiments aremerely illustrative of the technical principles of the present inventionbut not limitative of the invention. The spirit and scope of the presentinvention are to be limited only by the appended claims.

This application corresponds to Japanese Patent Application No.2008-156937 filed with the Japan Patent Office on Jun. 16, 2008, thedisclosure of which is incorporated herein by reference.

1. An MEMS sensor, comprising: a substrate; and a diaphragm made ofpolysilicon, the diaphragm being provided on the substrate and vibratingdue to an action of a physical quantity, the diaphragm integrallyhaving: a main portion arranged on the substrate in a floating state,and supporting portions extending in a direction along a surface of thesubstrate from two positions in a peripheral edge of the main portion,the supporting portions having a length not less than three times largerthan a width thereof, an angle formed by two straight lines connectingthe supporting portions and a center of the main portion with oneanother respectively being from 7.5° to 20° .
 2. The MEMS sensoraccording to claim 1, further comprising a first insulating film and asecond insulating film stacked on the substrate in this order, wherein aforward end portion of each supporting portion enters a space betweenthe first insulating film and the second insulating film, so that thesupporting portion is cantilever-supported by the first insulating filmand the second insulating film.
 3. The MEMS sensor according to claim 1,further comprising a back plate opposed to the main portion through agap on a side opposite to the substrate.
 4. The MEMS sensor according toclaim 3, further comprising a plurality of protruding upper stoppersprotruding from the surface of the back plate opposed to the mainportion toward the main portion for preventing contact between the mainportion and the back plate, wherein the plurality of protruding upperstoppers include a peripheral stopper opposed to the peripheral edge ofthe main portion and a center stopper opposed to a center portion of themain portion surrounded by the peripheral edge.
 5. The MEMS sensoraccording to claim 4, wherein a height of the peripheral stoppermeasured from the surface of the back plate is larger than a height ofthe center stopper measured in the same way.
 6. The MEMS sensoraccording to claim 3, further comprising a plurality of protruding upperstoppers protruding from the surface of the back. plate opposed to themain portion toward the main portion for preventing contact between themain portion and the back plate, wherein the plurality of protrudingupper stoppers have different heights measured from the surface of theback plate.
 7. The MEMS sensor according to claim 3, wherein athrough-hole is formed in the substrate, and the MEMS sensor furthercomprises a plurality of protruding upper stoppers protruding from thesurface of the back plate opposed to the main portion toward the mainportion for preventing contact between the main portion and the backplate, and the plurality of protruding upper stoppers opposed to aperipheral portion of the through-hole.
 8. The MEMS sensor according toclaim 1, wherein a plurality of protruding lower stoppers for preventingthe main portion and the substrate from adhesion are formed on a surfaceof the main portion opposed to the substrate.